Surface characterization of tungsten and tungsten carbide–cobalt probe materials for a fine‐pitch four‐point probe by variable excitation XPS using synchrotron radiation

In the course of the development of a new fine-pitch four-point probe, surface oxide layers of the probe material were characterized by a non-destructive depth profiling analysis with a variable excitation XPS using synchrotron radiation. The high surface sensitivity of this technique has allowed us to measure the ultra-thin thickness of the oxide layer on the probe material without damage. The XPS of W 4f of the tungsten metal was measured in the excitation x-ray energy range 100-1200 eV at the Photon Factory (BL-13C), Tsukuba, Japan and analysed by the simulation of attenuation in the layer models using the values of inelastic mean free path (IMFP) and effective attenuation length (EAL) calculated by the National Institute of Standards and Technology (NIST) database programs 71 and 82, respectively. The values of practical EAL at 1 nm depth were smaller than the IMFP by 20-40% and resulted in a smaller thickness of the surface oxide layer than for that of IMFP by about 20%. The result showed that the thickness of the surface oxide layer of the tungsten metal was 1.0 nm 'as received' and 0.5 nm after polish. The XPS of W 4f of the superfine grain (grain size < 0.5 μm) tungsten carbide-cobalt wires with 0.45 mm diameter, which is a probe size of conventional four-point probes, were measured and the results indicated that the thickness of the surface oxide layer of the tungsten carbide-cobalt was 0.4 nm 'as received', and the thickness of the oxide layer was increased to 0.5 nm after polish. The XPS of W 4f of the tungsten carbide-cobalt wire with 0.05 mm diameter of an actual probe for a new fine-pitch four-point probe system was also measured and the results indicated that the thickness of the surface oxide layer was 0.6 nm. The XPS of W 4f also indicated the existence of an intermediate thin layer of WO 2 between W and WO 3 .